1. Field of the Invention
The present invention relates generally to structural damping and vibration control devices and, more particularly, to passive piezoelectric shunt circuits.
2. Description of Related Art
Negative impedance converters have been used in the prior art for reducing or neutralizing an impedance of an electrical circuit. One particular type of negative impedance converter is a negative capacitance converter. A negative capacitance converter generally comprises an operational amplifier having a first resistor connected between the positive input terminal of the operational amplifier and the output terminal of the operational amplifier, and a second resistor connected between the negative terminal of the operational amplifier and the output terminal of the operational amplifier. A capacitor is connected between the negative terminal of the operational amplifier and ground. The impedance of the capacitor is equal the 1/j.omega.C. The input impedance of the negative impedance converter, measured between the positive terminal and the negative terminal of the operational amplifier, is equal to the negative of the impedance of the capacitor if the two resistors are made equal in value. Other configurations of negative capacitance converters can be generated by rearranging the two resistors and the capacitor.
Negative capacitance converters can be used in accordance with the prior art for providing passive piezoelectric shunting. Passive piezoelectric shunting is generally achieved by bonding on or imbedding in a structure a piezoelectric material, such as one or more thin patches of lead-zirconate-titanate (PZT). The structure will typically be subjected to vibration during normal use, and the thin patches of PZT material are implanted in the structure in order to provide structural damping and vibration control for the structure. As a result of the piezoelectric nature of the PZT material, approximately 60 to 70 percent of the mechanical energy of the vibrating structure can be converted to electrical energy. The electrical energy induced in the PZT material must be forced to flow as an electric current through an electrical shunting circuit. The electrical shunting circuit, including a negative capacitance converter and a resistor placed in parallel with the negative capacitance converter, is electrically connected to the PZT terminals in order to extract electrical energy from the PZT material, as the structure and the PZT material vibrate. The electrical energy is extracted and is dissipated in the resistor by Joul heating.
Since the impedance of the PZT material is generally capacitive with a capacitance value equal to C, a negative capacitance converter circuit having a capacitance value of negative C over the broad frequency band can be used to make the shunt circuit anti-resonant, i.e. cancel the reactive impedance, over the broad frequency band by connecting the negative capacitance in parallel with the PZT. This leaves only the resistive impedance or resistance in the shunt circuit. Negative impedance converters can provide a reasonable negative impedance for canceling the impedance of the PZT material, but the negative impedance converter of the prior art is not altogether effective. Negative impedance converters, when used with shunt circuits for canceling the impedance of a PZT material, often oscillate and, accordingly, cannot effectively cancel the impedance of the PZT material. A negative impedance converter can oscillate because the capacitance of the PZT material can change with frequency and temperature. The capacitance of the PZT material, in general, goes down with frequency. Any change in the capacitance of the PZT material will require a change in the value of the capacitor of the negative impedance converter, but the prior art negative impedance converters do not have means to reconfigure and stabilize the shunt circuit.